The present invention relates to the field of dissecting logs in a sawing installation. More specifically, the invention is related to a method and an apparatus for dissecting logs in such installation where a longitudinal, reciprocating conveyor is used for moving the logs relative to a double band saw assembly.
Such sawing installations are used for entirely dissecting logs into boards and beams along a plurality of reciprocal conveyor movements.
In the field of sawing installations various types of saws and sawing methods are used for converting a fresh log into a plurality of predetermined wood products, in particular boards and beams.
Besides reciprocating frame saws and circular blade saws one has also used various types of band saw assemblies for that purpose.
U.S. Pat. No. 3,318,347 discloses a method and an apparatus for multiple resawing utilizing a plurality of pairs of band saws being arranged on opposite sides of a longitudinal conveyor moving a log through the band saw assembly. By means of individual displacing units the various band saws may be positioned in a direction perpendicular to the conveyor direction. While doing so it is possible to entirely dissect the log in one pass only because the number of band saw blades used and their relative position to each other ensure that the log is dissected in so many wood products as desired. The thickness of the boards and beams so produced may be adjusted by displacing the band saw blades accordingly.
In this prior art installation the band saw blades are provided with sawing teeth only on one lateral edge thereof and the logs are fed through the band saw assembly in one direction only.
A similar band saw assembly is disclosed in German disclosure document 35 32 642 teaching, further, that the logs, prior to coming into engagement with the band saw blades, may be laterally flattened by means of appropriate chipping heads arranged upstream the band saw assembly.
German utility model 71 05 947, in contrast, teaches a recipro-cating conveyor band saw installation. In such an installation the log is affixed on a conveyor adapted to be reciprocally moved along a guide rail, thus moving the log through a band saw station and back again. The prior art band saw installation utilizes a band saw blade having also sawing teeth only on a lateral edge thereof. As a consequence, a wood product is sawn from the log only during a first conveyor stroke in a first direction whereas no such sawing action takes place when the conveyor is operated in a backward stroke taking the log back to its initial position. However, after each double stroke the band saw blade is forwarded by a predetermined amount in a direction perpendicular to the axis of conveyor displacement, the amount corresponding to the thickness of the next wood product to be sawn.
German utility model specification 94 04 583 discloses an endless band saw blade having teeth on both lateral edges thereof. However, only the teeth on one lateral edge are intended for sawing. The teeth on the opposite lateral edge are intended to remove dust or chips from the cutting area when, for example, a hand-held band saw is retracted after having made a cut into a piece of wood but not wholly through the wood. In such a case it may be that the saw blade is canted when being retracted because the sawn slit may be repleted with agglomerated saw dust or small chips. The rearward teeth on the band saw blade are, hence, only intended to remove such agglomerations of saw dust or chips but not for sawing.
One has, however, also used band saw blades having forward and rearward sawing teeth on opposite lateral edges in reciprocating band saw installations, as described before.
If a log shall be dissected by means of a band saw assembly having only one band saw blade, various configurations may be selected. The band saw blade may, for example, have teeth only on one lateral edge thereof. In that case, if the log shall be dissected into n boards, n+1 cuts are required. Further, the log after each pass has to be brought back into its starting position in front of the band saw assembly, either by means of a reciprocating conveyor or by means of another circulating device. From that initial position a second pass may be effected, sawing another board from the log.
As an alternative, one may, for example, provide chipping heads, as mentioned above, for flattening the log in an initial step on one side or on two opposite sides of the log. When doing so, one needs one cut or two cuts, resp. less for sawing n boards from a log. Therefore, if the log is initially flattened on both lateral sides, only nxe2x88x921 cuts are required.
As already mentioned above, one has also used band saw blades having teeth on both lateral edges. In that case one must move the log on a reciprocating conveyor. The reciprocating longitudinal conveyor may be configured as a reciprocating saw carriage as is well known in the art and comprising a carriage that is reciprocally moved along a corresponding guide rail.
The log is held on the carriage by means of clamps engaging the log on an upper longitudinal side opposite the side where the band saw blades come into engagement. These clamps to a certain amount also allow alignment of the logs on the carriage. When doing so, the time needed for entirely dissecting a log may be drastically reduced because the log may be sawn both during the forward stroke and during the backward stroke of the reciprocating conveyor, however, the number of cuts required remains the same even when in the above-mentioned alternate configuration a pair of clipping heads is used for laterally flattening the log prior to the first saw cut.
It is, further, well known in the saw mill technology to use so-called tandem-assemblies utilizing two band saw blades being distant from each other in a direction perpendicular to the conveyor axis. They are normally offset with respect to each other by the thickness of the board to be sawn. The two band saw blades are usually arranged in a staggered configuration, i.e. are also offset along the conveyor direction. If the log is fed through the tandem assembly, the outwardly lying band saw blade comes into engagement with the log first and the other band saw blade being at a somewhat more innerward position comes into engagement next.
For that reason it is hardly possible to use such tandem assemblies with double-teethed band saw blades. For, during the back stroke of the conveyor the log would first come into engagement with the inner band saw blade and then with the outer band saw blade which is highly disadvantageous for technological reasons.
It is, therefore, an object underlying the invention to improve a method and an apparatus of the above-mentioned kind such that the above-described disadvantages are obviated. In particular, the performance and throughput of conventional sawing installations shall be drastically improved.
These and other objects are solved by the inventive method and apparatus of the present application.
For that purpose, the invention proposes a method for dissecting logs in a sawing installation having:
a longitudinal conveyor adapted to be reciprocally moved along a first axis;
a first band saw blade extending along a second axis essentially perpendicular to the first axis, the first band saw blade having forward sawing teeth and rearward sawing teeth on a forward longitudinal edge and a rearward longitudinal edge, resp. thereof;
means for displacing the first band saw blade along a third axis essentially perpendicular to the first and the second axes;
a second band saw blade extending along a fourth axis essentially parallel to the second axis, the second band saw blade having forward sawing teeth and rearward sawing teeth on a forward longitudinal edge and a rearward longitudinal edge, resp. thereof;
means for displacing the second band saw blade along a fifth axis being essentially parallel to the third axis;
the method comprising the steps of:
a) placing a log on the conveyor;
b) affixing the log to the conveyor;
c) positioning the first band saw blade along the third axis to a first predetermined position on the third axis;
d) positioning the second band saw blade along the fifth axis to a first predetermined position on the fifth axis;
e) moving the conveyor along the first axis in a first direction, the first positions on the third and the fifth axis, resp. being selected such that the first and the second band saw blades effect two parallel cuts through the log by means of the forward sawing teeth;
f) positioning the first band saw blade along the third axis to a second predetermined position on the third axis;
g) positioning the second band saw blade along the fifth axis to a second predetermined position on the fifth axis; and
h) moving the conveyor along the first axis in a second direction opposite the first direction, the second positions on the third and the fifth axis, resp. being selected such that the first and the second band saw blades effect two further parallel cuts through the log by means of the rearward sawing teeth.
For the same purpose, the invention, further, suggests an apparatus for dissecting logs in a sawing installation having:
a longitudinal conveyor adapted to be reciprocally moved along a first axis, the conveyor comprising log receiving means and log clamping means;
a first band saw blade extending along a second axis essentially perpendicular to the first axis, the first band saw blade having forward sawing teeth and rearward sawing teeth on a forward longitudinal edge and a rearward longitudinal edge, resp. thereof;
first means for displacing the first band saw blade along a third axis essentially perpendicular to the first and the second axes;
a second band saw blade extending along a fourth axis essentially parallel to the second axis, the second band saw blade having forward sawing teeth and rearward sawing teeth on a forward longitudinal edge and a rearward longitudinal edge, resp. thereof; and
second means for displacing the second band saw blade along a fifth axis being essentially parallel to the third axis.
The object underlying the invention is thus entirely solved.
According to the invention, the radial positions of the band saw blades relative to the conveyor axis are interchanged during each change of direction of the conveyor. When doing so, a band saw blade having had an outer position during the last conveyor stroke will now assume the inner position and vice versa. As a consequence, during each stroke of the conveyor the outer band saw blade may first come into engagement with the log, as is intended for technological reasons.
The band saw assemblies may be provided with a variety of displacement units. For example, one may provide two entirely independent band saw assemblies having two likewise independent displacement units controlled by an appropriate circuitry in a push-pull operation. As an alternative, one may also use integral systems having a common drive motor for both band saw blades and a common displacement unit comprising an appropriate alternating transmission generating the required alternate displacement of the band saw blades.
In a preferred embodiment of the invention one may, as known per se, chip or flatten the logs at least on one lateral side thereof prior to making the first cuts. For that purpose one may use one or two opposite chipping heads as also well known in the art.
For dissecting logs into boards or beams one may use various cutting sequences. Within these cutting sequences it may be advantageous to rotate the logs by 180xc2x0 about the log longitudinal axis during a change of conveyor direction. When doing so, it is much simpler to let the band saw assembly engage the logs because the band saw assembly is normally located on one lateral side of the longitudinal conveyor.
It goes, further, without saying that within the scope of the present invention more than two band saw blades with appropriate change in position may be used. One may also use radial offsets between band saw blades corresponding to integer multiples of board thicknesses when displacing the band saw blades in a radial direction. Further, one may also displace the band saw blades by arbitrary amounts, for example when wood products of different thicknesses shall be produced, for example boards and beams.
Further advantages will become apparent from the subsequent description and the enclosed drawing.
It goes without saying that the features mentioned before and those that will be explained hereafter may not only be used in the particularly given combination but also in other combinations or alone without leaving the scope of the present invention.